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Multichiplet systems can be utilised to build a flexible, reconfigurable board-level computing platform with enhanced resource consumption and energy efficiency. Optical interconnects feature high IO capacity, distance-independent energy consumption, and low-delay connection. Optical linking fabric allows comprehensive customization of system architecture and resource allocation, which is not possible with electrical approaches. However, current optical technologies create static power overheads that reduce performance and dynamic power. This thesis offers and assesses unique architectural approaches for power-efficient multichiplet board-level optical connection usage. First, we recommend "unorthodox" UMA multi-chiplet architectures for on-board computers. We show it is only viable with optical interconnects, and simulations show it can improve execution speed and energy efficiency for a variety of workloads. We make our own simulation systems to evaluate the design trade-offs and rules for the network utilization methods are theoretically derived while the energy-delay products are investigated under various launch conditions. Then, we offer 'RENU,' a unique on-board optical interconnect control technique assessed with wavelength and spatial optical switching fabric. RENU maximises utilisation by rearranging interconnects depending on application network traces. Energy consumption is reduced, especially in low-use paradigms, compared to state-of-the-art. We then propose 'Min-ORUM', an online utilisation maximisation technique that reduces energy consumption for a variety of applications. Finally, we present 'SO-RA', a novel laser distribution control system using sophisticated SOA-based switch matrics. It reduces static power losses by reconfiguring in nanosecond time.

Nonlinear dynamical properties of semiconductor lasers have attracted considerable attention, and their rich behaviors enable many popular research topics. The research effort of this thesis has emphasized on two areas - one is photonic microwave generation based on period one dynamic of semiconductor lasers; the other is laser's chaotic dynamic. Microwave photonics has attracted considerable attention recently because of its practical applications in radio-over-fiber (RoF) communications links. A stable photonic microwave allows it to convey, in a cost-effective manner, wideband signals over optical fibers with low loss, large bandwidth and immunity of electromagnetic interference. Microwave photonics technologies consist of photonic microwave generation, processing, control and distribution. Many photonic microwave generation techniques have been proposed, which includes direct modulation, optical heterodyne technique, external modulation, mode-locked semiconductor lasers, optoelectronic oscillator (OEO) and period one (P1) dynamic of semiconductor lasers. Among these techniques, photonic microwave generation based on P1 oscillation dynamic has gained special attention due to its many advantages, such as: widely tunable oscillation frequency, and nearly single sideband (SSB) spectrum. The aim of this thesis in the photonic microwave generation area is to produce photonic microwaves based on P1 dynamic using low-cost vertical-cavity surface-emitting lasers (VCSELs). The technical contents in this area cover two parts. The first part is to generate broadly tunable photonic microwaves. Continuous tuning of the microwave frequency from 4GHz to up to an instrumentation limited 15GHz is experimental achieved through the adjustment of the injection power and the frequency detuning between the master laser and the VCSEL. Numerical simulations using a common spin flip model are also carried out, which agree qualitatively with the experimental results. The second part of the photonic microwave generation in this thesis is to explore effective approaches to not only reduce the linewidth but also improve the stability of the generated microwave. Due to spontaneous emission noise in the semiconductor laser, P1 dynamic inherently imposes phase noise, which increases the microwave linewidth of the generated microwave. This considerably affect the signal transmission performance of the modulated microwave signal in RoF applications. To address this challenge, single optical feedback and double optical feedback are applied in the experiments. The experimental results demonstrate that both single feedback and double feedback can reduce the linewidth of the generated microwave to about one tenth of linewidth without the optical feedback. However, single optical feedback may induce many side peaks due to external cavity frequency from the feedback cavity, the feedback phase needs to be carefully adjusted to suppress the side peaks. The side peaks can be suppressed by introducing the second optical feedback. The double optical feedback can also significantly enhance the stability of the generated microwave. The results of the numerical simulations are in good agreement with the experimental results. The other important dynamic of semiconductor lasers is chaos, which has attracted considerable research interest due to its many potential applications in secure communications, chaotic optical time-domain reflectors, chaotic lidars and physical random number generators. Optical feedback is the simplest method to generate chaos in semiconductor lasers, but a typical chaos generated by optical feedback has unwanted recurrence features termed time delay (TD) signature because of the optical round trip in the external cavity. The complexity, bandwidth and TD signature of chaos are the three main parameters for evaluating its applicability in abovementioned application scenarios. In order to find the correct operating parameters to achieve low TD signature and high complexity of chaos simultaneously, in this thesis, the influence of bias current and the feedback strength on the complexity and time-delay signature of chaotic signals in semiconductor lasers with optical feedback is investigated experimentally and theoretically. In the experiment, the effect of the data acquisition method on quantification of complexity is also examined. The experimental results show that the TD signature is approximately in an inverse relationship with the complexity of chaos when the semiconductor laser is subject to low or strong optical feedback. However, the inverse relationship disappears when the laser operates at higher bias currents with intermediate feedback strength. Numerical simulation based on Lang Kobayashi laser equations show qualitative agreements with the experimental results.

The aim of this project was to produce an integrate-able free space optical transceiver for Specks. Specks are tiny computing units that together can form a powerful network called a SpeckNet. The SpeckNet platform is developed by the SpeckNet consortium, which consists of five Scottish Universities and combines computer science, electrical engineering and digital signal processing groups. The principal goal of creating an optical transceiver was achieved by integrating in-house fabricated VCSELs (with lasing thresholds below 400 uA) and custom designed detectors on the SpeckNet platform. The transceiver has a very low power consumption (approximately 100 uW), which removes the need for synchronous communication through the SpeckNet thus making the network more efficient. I describe both static and dynamic beam control techniques. For static control, I used micro-lenses. I fabricated the lenses by greyscale electron beam lithography and integrated them directly on VCSEL arrays. I achieved a steering angle of 10 degrees with this design. I also looked at integrated gratings etched straight into a VCSEL and observed beam steering with an efficiency of 60% For dynamic control, I implemented a liquid crystal (LC) design. I built a LC cell with 30 individually controlled pixels, but I only achieved a steering angle of 1 degree. Furthermore, I investigated two different techniques for achieving beam steering by interference, using coupled VCSELs (a phased array approach). Firstly, using photonic crystals etched into the surface of the VCSEL, I built coupled laser cavities. Secondly, I designed and built bow-tie type VCSELs that were optically coupled but electrically isolated. These designs work by differential current injection causing an interference effect in the VCSELs far field. This technique is the first stepping stone towards realising a phased optical array. Finally, I considered signal detection. Using the same VCSEL material, I built a resonant-cavity detector. This detector had a better background rejection ratio than commercially available silicon devices.

Optical interconnects are potential solutions to meet the future data processor performance requirements. Optical interconnects have low crosstalk, and high bandwidth. Vertical cavity surface emitting lasers (VCSELs) are the crucial class of semiconductor lasers, which are manufactured on a single chip and are important components for supercomputers, datacenters, light-detection and ranging (LiDAR) systems, and optical short-reach interconnects. A VCSEL transversely integrated with multiple transverse-coupled-cavities (TCC) which demonstrates a 3-dB roll-off modulation bandwidth of 45 GHz, five times greater than a conventional VCSEL fabricated on the same epi-wafer, is demonstrated in this dissertation. In following, an on-chip platform for ultrafast electro-optical (EO) graphene modulator based on the Bragg reflector waveguide design is introduced. Compared with the conventional modulator design, this device enhances the light matter interaction in double layer graphene by more than 40 times, thereby avoiding the problem of power consumption. Due to the direct dependence of the applied voltage on graphene absorption, 5 dB E.R.) is expected. In following, a sub femto Joule, ultra-fast electro EO is conceptualized based on the adiabatic coupling in a three-level system. This design is implemented based on ITO modulator with three waveguides and sub-wavelength packaging which obtains superior performance over the current modulator techniques. By voltage-tuning a plasmonic mode via carrier modulation of an active ITO layer of the inner waveguide of the modulator, it is shown that tunability in and out of an adiabatic elimination (AE) region in the index design space enables an extinction ratio over 25 dB for just a 100 mV of applied bias. Also, in chapter 5, an on-chip Fourier-transform spectrometer on silicon-on-Sapphire is experimentally demonstrated. The spectrometer comprises an array of dozen Mach–Zehnder Interferometers (MZIs) with linearly increasing optical path delays between MZI arms. The retrieve of an optical spectrum is demonstrated with an inter-band cascaded laser centered at 3.3 μm. It is noted that a tradeoff between loss and geometry of components limits waveguide bend radius, detector size, and the number of MZIs that can be integrated on a photonic integrated circuits, defining the resolution of an spatial heterodyne Fourier transform spectrometer system. To this end, compressive-sensing techniques has been used to provide an intimidating path towards reducing the quantity of MZIs on a planar-waveguide chip.

III-N vertical-cavity surface-emitting lasers (VCSELs) show promise for numerous communications, lighting, display, and sensor applications due to their low threshold current, high beam quality, and arraying capabilities. Primarily, research has been focused on using c-plane based devices, but non-basal growth planes provide an interesting alternative due to a reduced quantum confined Stark effect; higher material gain; lower transparency current density; and inherent polarized emission. The anisotropic gain leads to VCSELs and VCSEL arrays where each laser is polarization locked along the a-direction. At UCSB, an m-plane VCSEL was first demonstrated in 2012 under pulsed injection and in 2018 under CW operation. Through that time, the device performance has improved and the polarization properties of the VCSELs has been experimentally verified. However, the wavelength of m-plane lasers is severely limited due to poor indium incorporation and high defect formation, inhibiting their adoption in many applications. This led to the question of how the benefits of using m-plane can be retained, such as the inherent polarization, while expanding the available wavelengths. The answer that was developed in this thesis is the use of a semipolar growth plane with higher indium uptake. After developing an epitaxial growth recipe and optimizing processing parameters for semipolar planes, we achieved the first demonstration of semipolar (20(21) ̅) VCSELs which were experimentally shown to be polarization locked along the a-direction and emit in the blue region. The devices had a 5λ cavity length, an ion implanted aperture, and a dual dielectric DBR design and showed an improvement in the differential efficiency, threshold current density and total output power relative to m-plane VCSELs with the same design. However, there were issues. The devices were only able to lase under pulsed operation, up to a 70% duty cycle. Focused ion beam images in conjunction with COMSOL modeling was used to identify the key structural features that contributed to the high measured thermal impedance. Nearfield images suggest that the LP01 mode was lasing near the edge of the aperture. This commonly observed spatial misalignment introduced additional sources of loss beyond the expected material absorption loss, including mode overlap with the implanted region and the metal contacts. The effect of these absorbing layers on the device performance relative to simulation models was estimated and highlighted the need for proper mode control.To improve the optical confinement, devices using a buried tunnel junction (BTJ) scheme to confine the current were fabricated and were found to lack the excess losses due to absorption seen on the initial semipolar samples. Significant filamentation was observed on these samples and several characterization methods, including optical and thermal nearfield images, were used to identify the source of the filamentation. Further comparison of multiple BTJ samples with different index guiding showed that the mode behavior was driven by the interplay of inhomogeneous current injection and index guiding. The cause of the inhomogeneous current injection is projected to be due to doping variations in the p-GaN but still requires further investigation for verification.

Wide-bandgap optoelectronic devices have undergone significant advancements with the advent of commercial light-emitting diodes and edge-emitting lasers in the violet-blue spectral region. They are now ubiquitous in several lighting, communication, data storage, display, and sensing applications. Among the III-nitride emitters, vertical-cavity surface-emitting lasers (VCSELs) have attracted significant attention in recent years due to their inherent advantages over edge-emitting lasers. The small active volume enables single-mode operation with low threshold currents and high modulation bandwidths. Their surface-normal device geometry is conducive to the cost-effective formation of high-density 2D arrays while simplifying on-chip wafer testing. Furthermore, the low beam divergence and circular beam profiles in VCSELs allow efficient fiber coupling. Nevertheless, GaN-based VCSELs are still in the early stages of development. Several challenges need to be addressed before high-performance devices can be commercially realized. One such challenge is the lack of high-quality distributed Bragg reflector (DBR) mirrors. Conventionally, epitaxial and dielectric DBRs are used which often involve complex growth and fabrication techniques. This dissertation provides an alternative approach where subwavelength air-voids (nanopores) are introduced in alternating layers of doped/undoped GaN to form the DBR structure. Selective electrochemical etching creates nanopores in the doped layers, reducing the effective refractive index relative to the surrounding undoped GaN. Using only 16-pairs, DBR reflectance >99.9% could be achieved. Several research groups have shown optically pumped VCSELs using nanoporous DBRs on c-plane. However, there are no reports of electrically injected nanoporous VCSELs. Using m-plane GaN substrates, we have demonstrated the first ever electrically injected GaN-based VCSEL using a lattice-matched nanoporous DBR. The nonpolar m-plane orientation is beneficial for leveraging the higher per-pass gain and polarization-pinning properties absent in c-plane. Lasing under pulsed operation at room temperature was observed at 409 nm with a linewidth of ~0.6 nm and a maximum output power of ~1.5 mW. This is the highest output power from m-plane VCSELs to date with relatively stable operation at elevated temperatures. All tested devices were linearly polarization-pinned in the a-direction with high polarization ratios >0.9. Overall, the nanoporous DBRs help in mitigating some of the issues that limit the performance of III-nitride VCSELs.

Vertical cavity surface emitting lasers (VCSELs) have been widely employed in short distance optical interconnects. Recently however a series of emerging applications are creating a rapidly growing demand for compact, low cost and high-performance light sources. 3D imaging and proximity sensing capabilities based on time of flight and structured light schemes are currently being deployed in a vast amount of consumer hand held devices, in particular in cell phones. Here VCSEL arrays of various sizes, densities and powers are being employed in volumes of multiple hundreds of millions per year. At the same time, wavelength tunable VCSELs with wide tuning range, fast tuning speed and single mode continues wavelength tuning are of tremendous interest for swept source optical coherence tomography (SS-OCT). This work investigates a series of VCSEL architectures that specifically address the requirements of the above-mentioned applications. Inhouse epitaxial growth by metal organic chemical vapor deposition was developed (chapter 2) to supply the wide variety of VCSEL structures investigated and in particular to enable the realization of devices that depend on regrowth steps. Optical pumping of high contrast grating (HCG) enabled devices was tested (chapter 3) as this method has proven successful in DBR based devices. Wavelength tunable VCSELs with a MEMS actuated HCG as sole top reflector where demonstrated (chapter 4 & 5). These devices showed single mode emission and continues electrostatic tuning covering a wide wavelength range. Thus, proving their concepts promising for application as sources in SS-OCT. Two different VCSEL architectures with lithographically defined current apertures have been investigated, one based on a buried InGaP heterostructure (chapter 5) and one based on a buried tunnel junction (chapter 6). Substituting the native oxide confined current aperture by a lithographic aperture of such kind enables a wealth of potential performance improvements. Improvements in reliability, thermal resistance, emitter uniformity, array density, high-power single mode emission and coherently coupled arrays can be reasonably expected of such devices. In combination with the HCG reflector lasing in a stable, single polarization can be achieved. At last, VCSEL arrays were fabricated at 850 nm and 940 nm wavelength in chapter 7.

Vertical-cavity surface-emitting lasers (VCSELs) are key optical components used in optical communication, datacom, and many other applications. Recently, as 3D sensing technology has become more popular, VCSEL arrays have attracted more attention. As a result, VCSELs with more advanced and improved performance are needed to satisfy the increasing trend. This is very hard to achieve by traditional VCSELs alone. Therefore, it creates an opportunity for VCSELs to integrate with other optical elements.Grating is such a traditional optical element but with many unique properties. It has been studied by researchers for a long time. There are many reasons why grating is an ideal candidate for such integration. Besides its various optical properties, its fabrication is usually very simple as well. Therefore, this thesis will focus on different ways that gratings can be integrated with VCSELs to bring novel and unique properties.First, we will introduce a design toolbox we developed to model such grating structures. With this toolbox, we will demonstrate a novel wavelength tunable VCSEL design. By replacing the traditional VCSEL top mirror by high-contrast grating (HCG) and changing the cavity design, the laser tuning range can be greatly extended. Second, we will discuss a more compact HCG VCSEL architecture by incorporating a low-index oxide spacer. By carefully optimizing HCG design, the highest efficiency HCG VCSEL has been demonstrated. Finally, we will introduce a two-step growth method to fabricate a new grating aperture VCSEL. By replacing the traditional oxide aperture by a grating, large-aperture, single-mode laser can be achieved. In addition, polarization selective emission is also possible. With these three examples, we hope to show the capability of integrating grating structures to VCSELs and pave the road for future VCSEL design.

Vertical-cavity surface-emitting lasers (VCSELs) are a special class of laser diode that use top-side and bottom-side parallel mirrors to emit a laser beam vertically from the top surface, as compared to conventional edge-emitting lasers that emit light from the sides of the devices. Compared to edge-emitting lasers, VCSELs have many unique attributes, such as a low threshold current that leads to low power consumption, high-speed direct modulation, superior beam quality, and they can be easily arranged into two-dimensional VCSEL arrays for scalable power. This leads to several exciting applications; for example, VCSELs are the key component in the Apple iPhone X that enables Face ID, which allows users to securely unlock their devices simply by looking at their phones. However, the VCSEL market is currently limited to red-emitting and infrared-emitting VCSELs that use GaAs-based and InP-based systems. If shorter wavelength emitting VCSELs could be created, it would open up a whole new world of untapped and exciting applications. For example, blue and green VCSELs could be paired with red VCSELs to create next-generation display and projector technology. The low power consumption and high beam quality of VCSEL-based displays would be particularly promising for virtual and augmented reality systems. Shorter wavelength VCSELs can be created using GaN-based materials, but these devices have been very challenging to create. The first GaN-based VCSEL was demonstrated in 2008, and only eight research groups have demonstrated these devices over the following decade. With the first report in 2012, the University of California, Santa Barbara (UCSB) has been the only group in the United States to create GaN-based VCSELs. Compared to the c-plane GaN VCSELs from all of the other groups, VCSELs from UCSB have used m-plane GaN, which uniquely provides 100% polarized emission for both individual VCSELs and VCSEL arrays. The main problem with GaN VCSELs from UCSB has been their inability to lase under continuous-wave (CW) operation. They could only lase under pulsed operation, which means that these VCSELs would fail if turned on for longer than a fraction of a millisecond. This has been a severe limitation that has prevented most practical applications of these devices. Therefore, the ultimate goal of the research described in this thesis has been to achieve CW lasing. This has been a tremendously difficult goal, and the initial VCSEL designs failed to lase, even under pulsed operation. Despite these initial discouraging results, this experiment was important because it led to an extensive failure analysis that revealed several key problems with the VCSEL design. Surface roughness prior to the DBR mirror deposition turned out to be a major problem that inhibited lasing due to scattering loss and reduced mirror reflectance. After performing experiments to improve the surface morphology, the surface roughness on the p-side was reduced by utilizing an indium flux during MBE tunnel junction regrowth, and the roughness on the n-side was reduced by removing an oxide residue that formed after the photoelectrochemical (PEC) undercut etch to remove the m-plane GaN growth substrate. Another significant problem was VCSEL yield in which only a small percentage of devices successfully transferred onto the flip-chip substrate, and most of those devices were cracked. A series of flip-chip bonding experiments were conducted to optimize the Au-Au thermocompression bond, but the yield only marginally improved at first. The flip-chip bond was also responsible for a severe thermal issue that prevented CW operation in previous devices. Based on thermal modeling in COMSOL, heat generated in the active region could not flow directly downward due to the thermally-insulating bottom DBR, so there was a bottleneck in heat transport through a relatively thin gold contact along the sidewall of the bottom DBR toward the flip-chip substrate. Focused ion beam (FIB) cross-sectioning revealed cracks in that thin metal contact, which were found to significantly impair the VCSEL thermal performance based on COMSOL simulations. Both the VCSEL yield and thermal performance were improved by implementing a new flip-chip bonding design. Instead of Au-Au thermocompression bonding, Au-In solid liquid interdiffusion (SLID) bonding was performed at a much lower temperature and pressure, which greatly improved the VCSEL yield. Furthermore, Au-In SLID bonding significantly improved the VCSEL thermal performance by incorporated a liquid phase during bonding so that the entire bottom DBR was embedded within metal. This led to the world’s first demonstration of CW operation for m-plane GaN VCSELs, and they were able to lase under CW operation for over 20 minutes of continuous testing.

III-N vertical-cavity surface-emitting lasers (VCSELs) can be used for various applications including data communications, displays, optical sensors, and atomic clocks. However, III-N VCSELs have never been commercialized because of difficulties in fabrication, reliability, and high power lasing. From past few years, we have intensively developed VCSEL performance as well as reliability. We started to get milliwatt-class VCSELs under pulsed operation along with the maximum output power of 2.8 mW. This advancement could be achieved by applying new aperture design and developing overall processing steps. We also investigated thermal characteristics of VCSELs. Then, a VCSEL with a metalorganic chemical vapor deposition (MOCVD)-grown tunnel junction (TJ) contact and a buried tunnel junction (BTJ) VCSEL were demonstrated for the first time. Finally, the maximum output power of 726 µW under CW operation was achieved.In the first chapter of this dissertation, motivations and applications of III-N VCSELs are presented along with introduction of VCSEL designs. In the second chapter, trials to optimize the VCSEL structure are demonstrated. Especially, the GaN TJ contact is intensely studied to replace a hybrid MOCVD/ molecular beam epitaxy (MBE) growth technique. Lastly, break down voltage of lightly doped p/n-GaN junction is verified for BTJ current aperture design. In the third chapter, processing steps are described for BTJ VCSELs and ion implant aperture (IIA) VCSELs. In the fourth chapter, optical and electrical characteristics of processed AII VCSELs and BTJ VCSELs are demonstrated and analyzed. Then, thermal analysis and effects were separately discussed. In the final chapter, conclusion and future works are mentioned.

This work was conducted to examine the mechanical stress occurring within Vertical Cavity Surface Emitting Lasers (VCSEL) due to packaging deformation under Continuous Wave (CW) operation. The modeling of a VCSEL device in this thesis utilized finite element analysis methodology to examine the viability and optimization of packaging options from a mechanical standpoint. Previous works have analyzed the resulting thermal stress within a single VCSEL structure, or from the standpoint of optical alignment. However, little to no work has been conducted to examine VCSEL arrays and device packaging from a mechanical stress perspective. This thesis demonstrates the resulting stress within the VCSEL array device at temperature extremes necessary for qualification in industry. Stress is found to be below documented fracture levels for two sub-mount packaging arrangements. The second sub-mount design includes copper through vias and proved to increase the overall stress in the package. Analysis of a VCSEL device bonded directly to a printed circuit board (PCB) found the use of plated through vias within the PCB to most viable. While package deformation induced larger stresses than previous designs, the VCSEL die and solder connections remained below fracture and shear failure levels. This thesis further demonstrates the most significant and cost effective method to reduce stress in any packaging variation is through depositing solder onto the sub-mount rather than the VCSEL die

Vertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from ~100 kA/cm2 to ~3 kA/cm2, while simultaneously increasing the output power from ~10 µW to ~550 µW. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs.

Integrated optoelectronics has demonstrated its great potential in numerous fields. In the past decade, its applications have rapidly expanded from the conventional long-haul optical communication to emerging areas such as data center, consumer electronics, energy harnessing, environmental sensing, and biological imaging. This revolutionary progress benefits from the advancement in light generation, manipulation, detection and its interaction with other systems. Device innovation is the key in this advancement. My dissertation discusses this innovation from two aspects: the integration of nanomaterials and the incorporation of metastructures in optoelectronic devices. The first topic focuses on material integration to facilitate on-chip optoelectronic devices. As microprocessors become progressively faster, chip-scale data transport has turned progressively more challenging. Optical interconnects for inter- and intra-chip communications are required to reduce power consumption and increase bandwidth. Integrating III-V compound semiconductors with superior optical proficiencies onto the silicon-based microelectronic backbone can pave the way towards a highly compact optoelectronic platform, combing the strengths of both materials. The direct growth of III-V micropillars on silicon substrate in the unique growth mode and under CMOS-compatible condition can yield single-crystal structures in sizes exceeding lattice-mismatched critical dimensions. These micropillars overcome the drawbacks of conventional nanowires, thus are endued with superior optical characteristics for on-chip lasers, photodetectors, and cost-effective solar cells. I will discuss the optical characterization of InP micropillars directly grown on silicon, and the optimization of their optical properties. The excellent material quality and device performance of these InP micropillars show promise for a variety of integrated optoelectronic devices. The second topic centers at the function integration enabled by high-contrast subwavelength grating (HCG), on the platform of vertical-cavity surface-emitting lasers (VCSELs). VCSELs are key light sources in integrated optoelectronics, with the advantages of low power consumption, low packaging cost, and ease of fabrication into arrays for wafer-scale testing. Mode-hop-free, fast and widely tunable VCSELs are also an ideal candidate for the emerging swept-source optical coherence tomography (SS-OCT) as well as light detection and ranging (LiDAR) applications. I will investigate how HCG, an ultrathin monolithic layer of sub-wavelength metastructure, can function as a highly reflective broadband mirror to facilitate lasing of VCSEL. Widely tunable HCG-VCSELs emitting at 1060-nm are demonstrated, a prevalent light source in SS-OCT for 3D eye imaging. Besides functioning as a tunable mirror, the HCG can also be designed as an integrated beam-shaping element at the same time. The rich properties and the large design space of HCG enable direct tailoring of the output beam features of VCSELs, such as transverse-mode control and far-field emission patterning with angular and spatial modulations by HCG. This opens new avenues for direct laser beam shaping with a monolithic optical element for integrated optoelectronics.

Over the past few years, tremendous effort has gone into the development of various optical building blocks for the silicon photonics platform. Part of the allure is that photonics circuits on silicon can be very small and leverage over 5 decades of scaling and existing microelectronics infrastructure. As such, photonic integrated circuits (PICs) on silicon are poised to address the ever-growing high-bandwidth needs of the servers and data centers of tomorrow, where the high-volume processing of a silicon platform and the low cost of traditional optical communications could redefine the constraints of high-performance interconnects.High contrast metastructures (HCMs) are an emerging solution for flat integrated photonic devices. These ultra-thin semiconductor ribbons can exhibit extraordinary properties that are atypical to traditional optical gratings, such as very broadband reflection at normal and shallow angles, high-Q resonances, and a readily engineered phase response. These properties have allowed for the design of a variety of novel optoelectronic devices such as vertical-cavity surface-emitting lasers (VCSELs), phase-arrays, lenses, and sensors. In this work, we explore three different types of novel HCMs on silicon. The first is a hollow-core waveguide that can channel light at wavelengths where traditional solid waveguides encounter difficulties; next is a silicon-based HCG VCSEL with the potential of being an efficient light source for silicon PICs; and lastly, an integrated wavelength meter, which can circumvent bulky off-the shelf instruments to provide wavelength characterization of guided light on a chip.

Complex optical pulse shapes are typically generated from ultrashort laser pulses by manipulating the optical spectrum of the input pulses. This generates complex but periodic time-domain waveforms. Optical Arbitrary Waveform Generation (OAWG) builds on the techniques of ultrashort pulse-shaping, with the goal of making non-periodic, truly arbitrary optical waveforms. Some applications of OAWG are coherently controlling chemical reactions on a femtosecond time scale, improving the performance of LADAR systems, high-capacity optical telecommunications and ultra wideband signals processing. In this work, an array of Vertical Cavity Surface Emitting Lasers (VCSELs) are used as modulators, by injection-locking each VCSEL to an individual combline from an optical frequency comb source. Injection-locking ensures that the VCSELs' emission is phase coherent with the input combline, and modulating its current modulates mainly the output optical phase. The multi-GHz modulation bandwidth of VCSELs updates the output optical pulse shape on a pulse-to-pulse time scale, which is an important step towards true OAWG. In comparison, it is about a million times faster than the liquid-crystal modulator arrays typically used for pulse shaping! Novel components and subsystems of Optical Arbitrary Waveform Generation (OAWG) are developed and demonstrated in this work. They include: 1. Modulators An array of VCSELs is packaged and characterized for use as a modulator for rapid?update pulse?shaping at GHz rates. The amplitude and phase modulation characteristics of an injection-locked VCSEL are simultaneously measured at GHz modulation rates. 2. Optical Frequency Comb Sources An actively mode-locked semiconductor laser was assembled, with a 12.5 GHz repetition rate, ~ 200 individually resolvable comblines directly out of the laser, and high frequency stability. In addition, optical frequency comb sources are generated by modulation of a single frequency laser. 3. High-resolution optical spectral demultiplexers The demultiplexers are implemented using bulk optics, and are used to spatially resolve individual optical comblines onto the modulator array. 4. Optical waveform measurement techniques Several techniques are used to measure generated waveforms, especially for spectral phase measurements, including multi-heterodyne phase retrieval. In addition, an architecture for discriminating between ultrashort encoded optical pulses with record high sensitivity is demonstrated.

Vertical-cavity surface-emitting lasers (VCSELs) are key optical sources in optical communications, the dominant source deployed in local area networks using multimode optical fibers at 850 nm. The advantages of VCSELs include wafer-scale testing, low-cost packaging, and ease of fabrication into arrays. The ease of array fabrication is particularly useful for space-division-multiplexed (SDM) links using multi-core fiber or fiber arrays. VCSELs emitting in the 1.3 m to 1.6 m wavelength regime, also known as long-wavelength VCSELs, are highly desirable for the rising applications of data and computer communications, in addition to optical access networks, optical interconnects and optical communication among wireless base stations. The potential advantages over conventional distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers include much lower cost due to smaller footprint and wafer scale testing, and significantly lower power consumption. InP-based long wavelength VCSELs have been demonstrated in the last 10 years. However, all the existing solutions require complex, expensive manufacturing processes. To date, long wavelength InP-based VCSELs have not made major inroads on the market. Designing a device structure that can be manufactured with as low cost as 850-nm GaAs-based VCSELs remains a major challenge.Tunable light sources are important for WDM systems with applications including sparing, hot backup, and fixed wavelength laser replacement for inventory reduction. They give network designers another degree of flexibility to drive down overall system cost. Such considerations are especially important for fiber-to-the-home and data center applications. Additionally, mode-hop-free and widely tunable light sources are a perfect candidate for high resolution laser spectroscopy and light ranging applications. Tunable VCSELs using micro-electro-mechanical structures (MEMS) are desirable because of their continuous tuning characteristics, making them promising for low cost manufacturing and low power consumption. Although many structures have been reported with wide, continuous tuning range, largely due to their fabrication complexity, low-cost tunable 1550-nm VCSELs have not yet been available on the market. High contrast gratings (HCGs) have emerged as an exciting new tool for achieving optical features such as broadband mirrors, planar lenses, and high quality factor resonators. Of particular use for VCSELs is a broadband mirror. These high contrast gratings in addition to acting as a mirror have features that can be exploited for many applications such as polarization differentiation and definable phase among others. HCGs are an exciting new tool for many optical applications. In this dissertation, I show how a high contrast grating can potentially solve several of the issues facing InP-based long wavelength VCSELs. First I introduce the state of the art in VCSEL research and portrait the trend of moving from short wavelength multimode VCSELs to long wavelength single mode VCSELs. Next I introduce high contrast gratings, discuss their implementation onto VCSELs, and demonstrate how they can be used to achieve more ideal modal qualities in the VCSEL, a single polarization mode as well as a larger area single transverse mode device. Then high output power, single mode, InP-based continuous wave VCSEL utilizing the unique properties of HCG is discussed. Following that discussion, I shift the focus to exploring the capability of these InP-based HCG VCSELs: a comprehensive model to optimize the electrical and optical design of the VCSEL device, high speed direction modulation, wide wavelength tenability and multiwavelength VCSEL array using the same platform. The high contrast grating may be an important tool to achieving higher performance VCSELs, and VCSELs with features that enable new applications.

Remarkable improvements in vertical-cavity surface-emitting lasers (VCSELs) have been made by the introduction of mode- and current-confining oxide optical aperture now used commercially. However, the oxide aperture blocks heat flow inside the device, causing a larger thermal resistance, and the internal strain caused by the oxide can degrade device reliability, also the diffusion process used for the oxide formation can limit device uniformity and scalability. Oxide-free lithographic VCSELs are introduced to overcome these device limitations, with both the mode and current confined within the lithographically defined intracavity mesa, scaling and mass production of small size device could be possible. The 3 μm diameter lithographic VCSEL shows a threshold current of 260 μA, differential quantum efficiency of 60% and maximum output power density of 65 kW/cm2 , and shows single-mode singlepolarization operation with side-mode-suppression-ratio over 25 dB at output power up to 1 mW. The device also shows reliable operation during 1000 hours stress test with high injection current density of 142 kA/cm2 . The lithographic VCSELs have much lower thermal resistance than oxide-confined VCSELs due to elimination of the oxide aperture. The improved thermal property allows the device to have wide operating temperature range of up to 190 °C heat sink temperature, high output power density especially in small device, high rollover current density and high rollover cavity temperature. Research is still underway to reduce the operating voltage of lithographic VCSELs for high wall plug efficiency, and the voltage of 6 µm device at injection current density of 10 kA/cm2 is reduces to 1.83 V with optimized mesa and DBR mirror iv structure. The lithographic VCSELS are promising to become the next generation VCSEL technology.

This dissertation details the work has been done on a novel resonant cavity linear interferometric modulator and a direct phase detector with channel filtering capability using injection-locked semiconductor lasers for applications in RF photonics. First, examples of optical systems whose performance can be greatly enhanced by using a linear intensity modulator are presented and existing linearized modulator designs are reviewed. The novel linear interferometric optical intensity modulator based on an injection-locked laser as an arcsine phase modulator is introduced and followed by numerical simulations of the phase and amplitude response of an injection-locked semiconductor laser. The numerical model is then extended to study the effects of the injection ratio, nonlinear cavity response, depth of phase and amplitude modulation on the spur-free dynamic range of a semiconductor resonant cavity linear modulator. Experimental results of the performance of the linear modulator implemented with a multi-mode Fabry-Perot semiconductor laser as the resonant cavity are shown and compared with the theoretical model. The modulator performance using a vertical cavity surface emitting laser as the resonant cavity is investigated as well. Very low Vπ in the order of 1 mV, multi-gigahertz bandwidth (-10 dB bandwidth of 5 GHz) and a spur-free dynamic range of 120 dB.Hz2/3 were measured directly after the modulator. The performance of the modulator in an analog link is experimentally investigated and the results show no degradation of the modulator linearity after a 1 km of SMF. The focus of the work then shifts to applications of an injection-locked semiconductor laser as a direct phase detector and channel filter. This phase detection technique does not iv require a local oscillator. Experimental results showing the detection and channel filtering capability of an injection-locked semiconductor diode laser in a three channel system are shown. The detected electrical signal has a signal-to-noise ratio better than 60 dB/Hz. In chapter 4, the phase noise added by an injection-locked vertical cavity surface emitting laser is studied using a self-heterodyne technique. The results show the dependency of the added phase noise on the injection ratio and detuning frequency. The final chapter outlines the future works on the linear interferometric intensity modulator including integration of the modulator on a semiconductor chip and the design of the modulator for input pulsed light.

Vertical cavity surface emitting lasers (VCSELs) have become of large commercial importance over the last two decades. They have become the dominant low cost, low power coherent sources in many applications such as optical sensing and short distance data communication.Despite their great success, there remain opportunities for improvement. Almost all commercially produced VCSELs are fabricated on structures based on a GaAs substrate. This restricts their wavelength range to between 650 and 1300 nm. Many potential applications exist at wavelengths outside of this range, so much work has gone into realizing VCSELs on other substrates. Though they have been realized, this has typically come with complexity that increases cost of producing such VCSELs. Of particular interest are InP-based VCSELs, which allow for emission at the wavelengths of 1320 nm and 1550 nm, two regions of high importance for optical communications applications.Another area of much research has been improving the modal characteristics of VCSELs. Typical VCSELs will lase in two polarization-degenerate optical modes. In addition, due to their structure, lateral optical modes also appear when a typical VCSEL's aperture becomes more than a few microns in size, limiting their output power. For high performance optical communications and sensing, a single mode laser is essential, so solving these mode problems is important.Next generation optical communication systems also require sources with an array of lasers of multiple output wavelengths or a tunable wavelength source that can be quickly tuned over a wide range of wavelengths. Schemes for both approaches have been implemented on VCSELs though various limitations of the current approaches have prevented them of being of large commercial importance to date. Tunable VCSELs have been limited in wavelength tuning speed by the thickness of their movable mirror. A commercially viable approach for an array of VCSELs of multiple wavelengths has also been difficult to achieve - most approaches shown to date require complex growth methods and the have not been able to achieve controllable wavelength spacing.High contrast gratings (HCGs) have emerged as an exciting new tool for achieving optical features such as broadband mirrors, planar lenses, and high quality factor resonators. Of particular use for VCSELs is a broadband mirror. These high contrast gratings in addition to acting as a mirror have features that can be exploited for many applications such as polarization differentiation and definable phase among others. HCGs are an exciting new tool for many optical applications.In this dissertation, we show how a high contrast grating can potentially solve several of the issues facing VCSELs and open new application spaces for VCSELs. First, we introduce the state of the art in VCSEL research and give a detailed overview of major areas of interest in VCSEL research. Next, we introduce high contrast gratings, discuss their implementation onto VCSELs, and demonstrate how they can be used to achieve more ideal modal qualities in the VCSEL, a single polarization mode as well as a larger area single transverse mode device. Then we show how the high contrast grating can be used in a tunable VCSEL to achieve ultra high speed tuning. Following that discussion, we shift our focus to implementing high performance, low cost VCSELs on InP with an HCG, showing two different approaches: a deposited Si HCG sitting on SiO2 and a monolithically-grown InP HCG. We conclude with a discussion of two approaches to achieve controllable arrays of VCSELs emitting at multiple wavelengths. The high contrast grating may be an important tool to achieving higher performance VCSELs, and VCSELs with features that enable new applications.

A microelectromechanical systems (MEMS) optical microphone that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm (JASA, v. 122, no. 4, 2007) is described. In the present embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. Advantages of this optical detection technique have been demonstrated with both omni-directional microphones and biologically inspired directional microphones. In efforts to commercialize this technology for hearing-aids and other applications, a goal has been set to achieve a microphone contained in a small surface mount package (occupying 2mm x 2mm x 1mm volume), with ultra-low noise (20 dBA), and broad frequency response (20Hz–20kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today, but would have noise performance characteristic of professional-audio microphones significantly larger in size and more expensive to produce. This paper will present several unique challenges in our effort to develop the first surface mount packaged optical MEMS microphone. The package must accommodate both optical and acoustical design considerations. Dynamic models used for simulating frequency response and noise spectra of fully packaged microphones are presented and compared with measurements performed on prototypes.

Abstract: There is a great deal of interest in monitoring atmospheric greenhouse gases such as methane. Although distributed feedback edge-emitting lasers are often used for atmospheric detection, the development of the vertical cavity surface-emitting laser (VCSEL) has allowed for an alternative source. The VCSEL exhibits several advantages over distributed feedback (DFB), edge-emitting lasers, especially in terms of power requirements and tuning capabilities. A second harmonic spectroscopy system based on a VCSEL laser is presented. Battery operation of the driver, temperature control and receiver is achieved. The system is used to detect methane gas in open path situations, as well as in gas cylinders. Temperature and current scanning are compared as methods for laser wavelength modulation. Mathematical methods for characterizing and filtering absorption signals are investigated. The receiver system is also used with a DFB laser to compare performance with the VCSEL. A software receiver using LabVIEW is implemented, and its performance is compared with the hardware designs. A minimum detectable limit of 1.4 ppm•m of methane for the hardware receiver is reported.

Abstract: The nature of work conducted during the course of study towards a MSc degree focused on tunable diode laser absorption spectroscopy (TDLAS). This field involves the in-situ detection of gas constituents from low concentration samples. Specifically, I will focus on TDLAS systems utilizing practical optics, readymade electronics, and commercially available near infrared vertical cavity surface emitting lasers (VCSEL). In attempting to lower the minimum detectable concentrations of constituent gases, quantifying contributory noise sources is vital. Consequently, I seek to characterize principle noise sources of a prototypical TDLAS system in order to gain understanding of the limits that inhibit detection of trace gas concentrations. The noise sources which were focused on can be categorized as follows: source laser noise, optical noise, and detection noise. Through this work it was my goal to provide the means of achieving superior sensitivities.

We have demonstrated a flexible optical waveguide film with integrated VCSEL and PIN photodiode arrays for the fully embedded board level optical interconnection system. One of the most critical issues in the fully embedded board level optical interconnection system is the signal beam coupling between the guided-wave structure and the aperture of VCSEL (or PIN photodiode). The coupling efficiencies of spherical mirrors are calculated as a function of mirror radius. The optimum mirror radius ranges which are compatible with the fully embedded board level optical interconnection system are theoretically verified. The thermal characteristics of a thin film VCSEL are studied both theoretically and experimentally. The thermal resistances of VCSEL with variable thickness, ranging from 10 [mu]m to 200 [mu]m, have been determined by measuring the output wavelength shift as a function of the dissipated power. The thermal simulation results agree reasonably well with experimentally measured data. From the thermal management point of view, a thinned VCSEL has an exclusive advantage due to the reduction of the thermal resistance. The thermal resistance of 10 [mu]m thick VCSEL is 40 % lower than that of 200 [mu]m thick VCSEL. The theoretical analysis of thermal via effects is performed to determine optimized thickness ranges of thin film VCSEL for the fully embedded structure. Thermal resistance of the fully embedded thin film VCSEL with closed and open thermal via structures are also evaluated with the suitable VCSEL thickness reported. The high-performance computing system is demonstrated using a 16-channel optical backplane using thin film volume holographic gratings. The optical backplane contains TO-46-Can-packaged VCSELs and photodiodes as an optical transmitter and receiver, respectively. Optical packaging plates are fabricated for 4 X 8 array packaging for 16-VCSELs and 16-Photodiodes. Packaging issues including crosstalk and alignment tolerance are studied to design a low cost optical packaging scheme. Thin film volume hologram grating is fabricated on glass substrate to redirect light beams. An individual single channel performs at a 100 MHz data transfer rate. The high-performance computing system using 16-channel optical backplane is demonstrated at a 1.6 Gbps data transmission.

It is possible to grow defect-free strained layers on patterned substrates (mesas or grooves) up to thicknesses far exceeding the critical thickness. Defect nucleation and propagation are inhibited in such growth. This property was exploited to design, fabricate and characterize a novel 1.55mum InP-based patterned VCSEL with lattice-mismatched GaAs/AlGaAs top Distributed Bragg Reflectors (DBR), with defect density less than 104 cm -2. TEM and photoluminescence studies were used to evaluate the optical quality of the grown material. Lasers were designed and fabricated with InP/InGaAsP bottom mirrors, laterally oxidized InAlAs current confining layers, and a short-stack, high-contrast GaAs/AlxO top DBR mirrors. 8--40mum diameter VCSELs have been characterized. A threshold current of 5mA is observed at 15°C for an 8mum diameter device; and up to 60muW of light output was recorded. The oxidation of InAlAs in InP-based heterostructures was studied for long wavelength VCSEL applications. The dependence on adjacent layer composition, thickness and doping were investigated. Several key contributions were made in this area, including: the characterization of the process at lower temperatures reduces substrate damage, observation of a dependence on adjacent layer composition resulting in the highest oxidation rate for InAlAs, and the first demonstration of an oxide-confined InP-based VCSEL. GaAs-based VCSEL arrays with AlGaAs/InGaAs active region for high power applications were designed and fabricated using a planar. Individual devices as well as arrays of varying size were characterized and operated with maximum conversion efficiencies of 5%. Good uniformity in threshold current and scalable power were obtained for a variety of arrays. 10 x 10 arrays, operating with 3.5% conversion efficiency, gave powers in excess of 60mW (limited by the current supply). The thermal crosstalk in 4 x 4 VCSEL arrays was also investigated. A thermal isolation of 2mW/K was recorded for devices separated by 50mum; and a conductivity is 0.057W/cm-K is extracted from the data. The ouput power of the lasers is temperature-insensitive under constant voltage operation. The multi-transverse mode behavior of a 2 x 2 array was studied. We believe that spatial hole burning and non-uniform current injection are the mechanisms responsible for the multi-mode characteristic of the arrays.